ESDU 08012

ESDU 08012 provides information on the use of passive devices for acoustic suppression in rectangular planform cavities. The very wide range of passive devices employed in both wind-tunnel and flight testscan be separated into seven main types. In the discussion of the various types tested, even the less successful ones are mentioned, since ineffectiveness in one situation should not be taken to implyineffectiveness in all situations - although that may be true of some. The particular types of passive device covered are fences or spoilers, vortex generators, front and/or rear wall geometry changes, the rod-in-crossflow, passive resonance tubes, Helmholtz resonators, and baffles and absorptive materials. The work is prefaced by a consideration of the various means whereby passive devices affect the cavity shear flow. Thedevice types on which a considerable amount of research has been carried out are tackled in a common fashion involving firstly a historical overview of the research, secondly an assessment of the effectiveness of the device in terms of acoustic suppression, thirdly whatever can be said concerning drag, and finally a summary of overall conclusions. The device types which have had relatively little research attention are dealt with in a more general way, with conclusions being given where possible. The Item concludes that carefully designed spoilers and rear wall ramps have been found to be successful cavity noise suppression devices atsubsonic speeds, and satisfactory at low supersonic speeds, as evidenced by their widespread past and present use on military aircraft. The newer rod-in-crossflow concept shows considerable promise in modal and background noise alleviation over a wide Mach number range. Of the other devices, the small amount of available data suggests that rearward-facing steps ahead of the front wall, vortex generators, Helmholtz resonators and acoustic liners could be successful in appropriate situations, or perhaps in combination with another device. The particular question of the intrinsic drag of the various devices and the overall drag of a cavity before and, especially, after suppression requires more research.